In recent years, thin-film transistors (TFT) using an oxide semiconductor film of InGaZnO as an oxide of indium (In), gallium (Ga), and zinc (Zn) or the like have been developed. The InGaZnO thin-film transistor can be fabricated easily even at low temperature and the mobility thereof is known to be high at 10 cm2/Vs or more. Thus, the realization of BEOL (back end of line) transistors embedded in multilayer interconnection is expected. In addition, the application thereof to the next-generation liquid crystal panels is expected.
In an InGaZnO thin-film transistor, a Schottky junction is formed between InGaZnO and a source or drain electrode (for example, molybdenum (Mo) or tantalum (Ta)) due to a work function difference. Thus, when compared with an ohmic junction, contact resistance increases. In a transistor in which the gate length is, for example, about 10 μm or more, the ratio of channel resistance is sufficiently larger than that of contact resistance and the contact resistance poses no major problem of affecting the drain current. However, with a decreasing channel length accompanying finer patterns, the ratio of the contact resistance increases and a problem of difficulty to secure a sufficient drain current due to the contact resistance may be caused.